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James Benford: Comments on METI

Pardon this extended introduction to Jim Benford’s response to Nick Nielsen’s Friday essay, but it comes at a serendipitous time. Jim’s recent online work has reminded me that we in the interstellar community need to work to see that as many resources as possible are made available online. In the absence of specialized bibliographies, useful information can be hard to find in more general indices. And it’s always dismaying to read an intriguing abstract only to realize that the paper itself is behind a pricey firewall. Access to academic libraries certainly helps, but online databases still vary in what they make available, which is why I always check the home pages of the authors of a given paper to see if they have posted a copy of their work themselves. Scientists can do much to get the word out, as Jim’s new site attests.

You’ll find it at http://jamesbenford.com/. Over the weekend, after Nick had discussed METI (Messaging to Extraterrestrial Intelligence) on Friday, I resorted to Jim’s site to pull the Benfords’ key papers on cost-optimized interstellar beacons back up on screen. Preprints of these are available on the arXiv site, but how much more useful to have a single site with a researcher’s papers not just in preprint but in final form. Jim and brother Greg’s work on how beacons might be built, what their broadcasting strategies would likely be, and how we might go about finding them is moving SETI in interesting directions and it’s all here in one place.

So are references to Jim extensive work on microwave beaming to sailcraft, on fusion and pulsed power and plasma physics at large. Here we’re still limited to bibliographic entries because of publishers’ policies, but the bibliography is itself a valuable tool, and we can hope, as publishers gain more experience with networked resources, that open access to scientific work (particularly that funded by taxpayers!) will increasingly become the norm. Interstellar studies needs good bibliographers and researchers can help by cataloguing their own work.

But enough of this extended introduction to the response Jim sent to Nick. What to do about sending messages to other stars? For that matter, just how far are our own electromagnetic signals traveling? We have much to learn as the investigation continues.

by James Benford


I’m very pleased to see Nick Nielsen’s essay. Fits my attempts to widen the discussion of METI. METI is an issue that really should be debated further; as it is increasingly possible that someone will announce us. These are not easy questions; they are ultimately social questions, so should be widely discussed.

I would suggest another paper of mine which treats METI directly. It’s in the Special Issue of JBIS on the METI Debate that I am editing:

“Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, in press, JBIS (2014).

A draft version is on my new website, at

A few comments:

1) Opponents of METI do not ‘maintain that the “leakage” of the ordinary (unintentional) EM radiation of a technological civilization cannot be detected at interstellar distances”. No, they say just that past signals are undetectable using our present and projected technology. Future leakage may well be larger – see the following point.

2) Several of the responses to my quantitative arguments showing that neither Earth-scale radio telescopes or even the Square Kilometer Array (SKA, that radio astronomers hope to build, but have yet to find the funding for) are able to detect leakage radiation or messages from our radio telescopes only at very limited range of at most a few light years. Their responses simply say that ETI will have far larger radio telescopes as big as Chicago or New Hampshire.

It’s not that easy. Such assumptions are not without implications. The proper scientific thing is to consider the implications of such assumptions. Larger-scale civilizations will leave larger footprints.

If we use Claudio Maccone’s statistical argument that results in the nearest ET civilization being roughly 1000 light years away, no message we can transmit from Earth within the limits of our technology would be detected unless the receiving civilization were substantially wealthier than us, by a factor of 1,000. The details: If we use the ability to build larger area radio telescopes as an indicator of the scale of civilization, that means the energies consumed, and so the wealth, should be larger by a similar amount.

Are there observable features of such civilizations? I think so, especially if they are nearby. If there were a civilization at Alpha Centauri with 1,000 times the energy consumption of earth, they would be able to do extraordinary things. They might modify their climate, beam power around their solar system, construct and launch starships, such as the ‘sailships’, beam-powered sails that Project Forward is conceptualizing. They would be observable.

So just imagining that ET is far more powerful than we are is not a simple escape clause from the arguments I made.

3) My work with John Billingham shows what METI would cost us: ~ 10 B$ for 1000 light year range. Countering METI would cost more than METI, but of same order. To suppress METI, radiate the same signal exactly 180° out of phase so that it cancels, suppressing the message.

4) “Energy and material limitations will cease to be relevant for all practical purposes.” That’s doubtful, as economic history shows. I remember when nuclear power was going to make electricity ‘free’. Didn’t happen. What does happen is prices for some things fall. For example Al, rare before 1800, now used for Coke cans.

5) Spectroscopy of exoplanet atmospheres could reveal markers of life like oxygen, but markers of civilization are very hard to see. Can anyone give an example of something detectable? CFCs aren’t, they’re too small.


Comments on this entry are closed.

  • Brett Bellmore November 11, 2013, 11:21

    “To suppress METI, radiate the same signal exactly 180° out of phase so that it cancels, suppressing the message.”

    A classic case of “easier said than done”. Short of assuming cooperation from the METI transmitter, how are you going to maintain a reliable phase lock? And assuming active non-cooperation, preventing such a phase lock would be quite easy, simply requiring occasional random phase shifts. At times the attempt to suppress the signal would actually be increasing it’s power!

    I think the arguments against METI are rather weak, except for the simplest: At present, we have better things to spend the money on.

  • Ron S November 11, 2013, 12:19

    “To suppress METI, radiate the same signal exactly 180° out of phase so that it cancels, suppressing the message.”

    Aside from the huge technical challenge of meeting the phasing requirement, do you really believe you will have the exact required signal structure and content from a hostile party to even attempt this? Or perhaps you only intend this to be done for “cooperative” transmitters?

  • Craig Watkins November 11, 2013, 12:54

    RE: detectable signs of civilizations
    There is a project being proposed called Colossus which aims to detect the heat signatures of civilizations within a 50-75 light year range.

    In general, assumptions about alien technology, especially an alien civilization even with even a few thousand years of scientific and technological advancement past our own seem hard to make with any confidence.

  • Markham November 11, 2013, 13:04

    I have often wondered about the interstellar optical signature of an advanced active interplanetary CIV.
    Some assumptions have to be made about what constitutes “advanced” propulsion but since most we know of require massive energy production and expulsion at non-natural frequency/energy levels what would optical seti make of such a solar system?

    Would brief fusion burns aligned in our direction appear as bright brief flashes that could be scanned for?
    Could the signature of laser pumped sails (when the beam coincidentally aligned with us) be observed?

    If most of the interplanetary propulsion methods we envision produce optical (or radio) signatures why are we focusing on METI or CIV energy leakage so much. Wouldn’t a CIV engaging in active mining of their interplanetary resources be visible at significantly greater distances than an energy leak only CIV?

    I wonder at what distances the various methods of interplanetary propulsion could be detected…

  • Tarmen November 11, 2013, 13:48

    In our oceans, it is only the whales and dolphins that sing out to their hearts ‘ content. The multitude sorts of fish are quiet for good reason. Many strange thing are lurking under rocks. And the old old shark-kind are quiet until their jaws snap.
    We should stay quiet until we know for sure, that we are dolphins or sharks, and not just another fish.

  • David Cummings November 11, 2013, 13:58
  • Michael November 11, 2013, 14:30

    May be dust created by asteroid mining could be detectable. Perhaps even large artificial worlds orbiting their stars in our line of sight causing transiting dimming and/or strange starlight reflectance’s could point out a technological civilisation.

  • Wojciech J November 11, 2013, 15:49

    “Spectroscopy of exoplanet atmospheres could reveal markers of life like oxygen, but markers of civilization are very hard to see. Can anyone give an example of something detectable?” Sometimes the most obvious answer is the one we have most difficulty to see.
    City lights.
    Not spectroscopy but advanced hypertelescopes would be able to take small images of planets with night lights visible. Perhaps even images of continents in resolution high enough to see urban sprawls and changes to vegetation.

    As to advanced civilizations(more than ours):space drives, space elevators, space solar power satellites etc.

  • Wojciech J November 11, 2013, 15:54

    Markham-this might be of interest to you:
    Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft
    Zubrin, R.

    Centauri Dreams posted about this in the past.

  • Adam November 11, 2013, 16:46

    Hi All
    Good suggestions on technological artefacts that *might* be visible. I think almost all of them have been discussed in the literature. The idea of transiting mega-structures is an interesting one as there’s Karl Schroeder’s “Virga” concept – a vast air-filled habitat – which might prove attractive to old “Super-Civilizations” wanting to rearrange their natural planets. One question is the limits of scale of such structures – material strength fails long before any other physical limits are reached, such as the Virial Theorem’s limit on collapsing or expanding self-gravitating balls of gas. But what about self-supporting structures? If an enclosing shell is supported by internal gas pressure, then the main structural stresses are removed, though tidal forces will need to be compensated for.

    Just how big can such “Air-Shells” get?

    A suggested upper limit that I’ve discussed with Karl is the size at which the weight of the enclosing shell matches the air-pressure of the gas enclosed and the enclosed air’s own self-gravitational weight matches its internal pressure at its core. For a gas mix at Earth pressure, temperature and density, the limiting size is ~22,000 km radius. At that limit the mass of the shell and the gas are equivalent and much, much less than a natural planet of the same size. Using gas mixes of lower densities – such as breathable hydrox mixtures – the upper size limit is even higher. Jupiter-sized gas enclosures could be created but massing only as much as the Earth or the Moon.

    Thus we should study with interest any transiting “planets” which are anomalously under-dense.

  • Markham November 11, 2013, 17:33

    Thanks for the link to the article, I would not have expected such short detection distances for the high-powered transport technologies such as the 100 LY limit for a chance nozzle alignment (anti-matter). A mag sail being the most detectable out to several thousand LY was also not expected.

    That leaves me wondering what the detecting distance for chance alignments of deep space laser communication would be.. based on the interplanetary propulsion results probably not promising.

    There is also the additional planetary signature of asteroid detection radars and space defense radars that would contribute to a METI discussion. Here’s a sample of active radar detected asteroids http://echo.jpl.nasa.gov/~lance/radar_detected_neas_summary/asteroid.radar.history.jpg
    I have no idea how frequent or powerful those outwardly directed radar’s are compared to METI beams.

  • Wojciech J November 11, 2013, 18:20

    Adam-another interesting concept.
    A bit off-topic digression-the amount of ideas of things we can search for and detect will certainly change the face of SETI and astronomy-I am sure. Even if it will not happen immediately, eventually I believe exoplanet research will start dominating the field. Especially if we get new space observatories and space telescopes. Amazing times.

  • David Cummings November 12, 2013, 7:48

    Adam, could we build such an air-shell ourselves, around Mars? Of course not with today’s technology, but sometime in the next century I’m hoping this will be feasible. The problems I see with an air-shell involve centering and border control. How would we keep an air-shell centered around the planet, maybe tethering with a series of space-cables around the planet’s equator?

    And how would we permit entry and exit and at the same time deal with meteor strikes.

    This is an extremely interesting subject with the payoff being a high-density atmosphere on Mars.

  • Alex Tolley November 12, 2013, 12:17

    And how would we permit entry and exit and at the same time deal with meteor strikes.

    Good questions. My 2 cents:
    1. Meteors. They will punch through the shell, but that does not mean explosive decompression, just a slow leak. Build it so that it self repairs, e.g. sliding sheets to cover the hole[s] and repair bots to make the repair permanent.
    2. Access. Have an airlock on the ground, with a tube to access vacuum. The tube is made of the same light material as the shell, and the shell supports the tube so that you do not need a strong compression structure reaching to the shell surface.

  • Brett Bellmore November 12, 2013, 21:39

    Mars is known for having at least one remarkably tall mountain. Put your space port on top of Olympus Mons, and the top of your atmospheric shell below that top.

    The problem is, restraining that shell requires it to be quite heavy, or held down by a lot of really strong cables. Imagine the planet surrounded by a balloon, with a forest of cables anchored in bedrock.

  • Daniel November 12, 2013, 23:07

    2. Access: Would a space elevator with an airlock be workable? Then you wouldn’t need worry about landers missing the tube.

  • Mike November 13, 2013, 0:15

    To Adam, when you describe the Air-Shell’s gaseous interior you mention hydrox. Is that a mixture of oxygen and hydrogen? If so a planet sized engineered atmosphere composed of hydrogen and oxygen would be a rather difficult place to obtain a good fire insurance policy.

  • James Benford November 13, 2013, 12:07

    Ron S : I didn’t say it would be easy. If the location is known, the direction can be sensed by an array of sensors around the site. Because the message bandwidth will be narrow to compete against the background noise, the data rate would be limited. So a slightly late counter-broadcast will still cancel out the message. This is economical, as you’re broadcasting in only one direction.

    But this is just one form of jamming, which has been practiced for 70 years. It’s part of the larger subject of electronic warfare (EW). That’s an entire discipline, and the US and others has great capability to introduce measures against a specific radiator, including phasing. There will of course be counter-measures, counter-countermeasures, etc. Been going on for many decades. So the tech already available will enable jamming of rogue radiators.

    The hard issue is how to do this at high powers. Phase-locking of such GW level sources my group solved back in the 80’s. (See my site>papers>high power microwaves for the relevant papers.)

    Markham points out that fusion rockets & beam-driven sails would have signatures. That’s my point: higher civilizations will have larger footprints. Where are they? Of course, there are many transient events observed in astronomy, but do not recur or are not even looked for.

    Michael: Industrial asteroid mining would compete with the substantial signature of the zodiacal light, caused by dust in the ecliptic plane. Some systems have a lot of dust relative to ours, so it’s probably not a useful signature.

    Wojciech J: “City lights.
 Not spectroscopy, but advanced hypertelescopes would be able to take small images of planets with night lights visible. Perhaps even images of continents in resolution high enough to see urban sprawls and changes to vegetation.”
    I’ve seen this idea of looking for city lights, but have never seen it sufficiently quantified. To be serious in such ideas, I keep saying, one must quantify! So I did a rough estimate. Of course, if you look directly at the night side of the exoplanet, you see its star. So you look off at it when it’s off at an angle, but then the exoplanets star-illuminated crescent grows, so there’s a signal-to-noise tradeoff between city light signal and the exoplanet‘s reflected starlight. That’s the scale of your ‘hypertelescopes’, which I calculate to be roughly 10 km aperture in the visible. At current prices, that would cost on the order of the Earth’s annual GDP. Daunting unless they’re a supercivilization, and we’re back to observable issue, as above: Where are the observables of this nearby supercivilization?
    Adam: Good idea about the ‘gas enclosures’!

    Markham: For a treatment of chance alignments and planetary radars see, toward the end, the piece by Billingham & myself that I referenced in my initial piece, viewable on my website.

  • Ron S November 13, 2013, 12:50

    James: You’re right, it isn’t easy. If you assume a restricted signal as you suggest (not broadcasting music to AC as some seem to enjoy doing), you *may* be able to feedback the sensed signal to keep the phase shift low. Even so the challenges go well beyond this. For example, phase isn’t good enough. You also have to match amplitude. This is more difficult than phase locking (if you can achieve that). A small difference in amplitude can result in an attenuation of no more than 1 db.

    Also, as you would know, the energy of both transmitters does not really cancel. Just, perhaps, in one very tiny patch of sky. Conservation of energy and all that. However this may be acceptable.

    A brute force jamming signal is, as you suggest, just another type of signal for ET to receive. All it does is submerge the modulation of the jammed signal.

    I seriously doubt any government would bother to undertake the cost of jamming even if they had the motivation (for whatever policy reason). If they have the motivation (which I think very unlikely) there are more…uh…direct means available to governments.

  • Chris Winter November 13, 2013, 13:24

    We’ve gone from gaslights in city centers to essentially planet-wide electrification in little more than a century. Assuming a supercivilization had been observing our solar system over that time, they might have been able to conclude based on the diminishment of darkness on Earth’s night side that artificial lighting was responsible. Granted it would be difficult, but I am not convinced it’s impossible.

  • Markham November 13, 2013, 21:58

    James: Thanks for the link on planetary radars and chance alignments. That article would also suggest even if we went to laser based interplanetary probe communication there would be significant challenges in intercepting such narrow beam (even with dispersion taken into account).

    This pretty much leaves something like a continuous sky survey designed to locate transient signals, take a spectroscopic sample, and alert other equipment to examine that transient. Characterization would have to happen even faster than we do with gamma burst or supernova analysis.

    Would a long fusion burn have a spectroscopic signature that could easily be noted as artificial by automatic survey? I know we use humans in the loop for the interstellar zoo catalog projects so maybe automatic is out for now.

  • Gregory Benford November 15, 2013, 8:47

    Those who commented on economics in this and earlier posts: this is
    just the same old Altruistic Alien Argument we discussed in

    It assumes some transcendent economy where nothing costs anything,
    like the fantasy novels of Iain Banks’ Culture. (He had to find
    various irrational reasons for conflict, since there was no struggle
    for resources. No money, either, a la Star Trek. Pure nonsense to any

    The Altruistic Alien Argument implied that the beacon builders will be vastly
    wealthy and make everything easy for us. We’ve tested that for half a
    century: no results. Indeed, since the Altruistic Aliens have no
    theoretical bound, you must explain why there’s no bright neon sign in
    the sky saying WE’RE HERE, FOLKS that everybody in the galaxy can see.
    (Any why skimp? All other galaxies, too.)

    Consider our own time. We’re millions of times more rich than cultures
    of thousands of years ago. (I’m writing this in Petra, Jordan, which
    left carved sandstone tombs, the only reason we know that impoverished
    time.) How come there’s still a vexing struggle for wealth, since costs for
    resources have shrunk? Because labor still costs. It always will.
    People need jobs.

    You can imagine robots who carry out your every wish (Jill Tarter
    tried to counter our economic focus by saying, “Maybe they’ll have
    slaves!” I’m not making this up.) But slaves cost something too, and others
    will want them to do something else.

    I think the Great Silence implies a lot of bad things: life is rare;
    life is strange and maybe hostile (so shut up); SETI is too far from alien
    lives to be worth the price–and many combinations of the above. So
    METI is a risk of unknown size. Prudence should rule.

    Further, we should frown at attempts to profit from people who would
    “send your message to the stars!” style advertising. This not only
    cheapens the entire field. It also defrauds the public, as Jim and
    others have shown by direct calculation. (Numbers matter!) METI could
    drive away support for SETI.